Java Comments

Why comments matter

A comment is text the compiler skips entirely. Comments do not affect how a program runs; they exist to help people, including your future self, understand why the code was written a certain way. Good comments explain intent, not just repeat what the code obviously does.

Single-line comments

A single-line comment begins with two forward slashes. Everything from the slashes to the end of that line is ignored. You can place one on its own line or at the end of a line of code.

public class Main {
  public static void main(String[] args) {
    // This line greets the user
    System.out.println("Hello, World!");
    System.out.println("Done"); // a trailing comment
  }
}

Multi-line comments

When a note spans several lines, wrap it between /* and */. Everything inside those markers is ignored, no matter how many lines it covers.

public class Main {
  public static void main(String[] args) {
    /*
      The lines below print a short greeting.
      This whole block is ignored by Java.
    */
    System.out.println("Hello, World!");
  }
}

Using comments to disable code

Comments are also handy while testing. By turning a line into a comment, you can temporarily stop it from running without deleting it. This is often called commenting out a line.

public class Main {
  public static void main(String[] args) {
    System.out.println("This line runs");
    // System.out.println("This line is skipped");
  }
}